Electron transfer between neptunium and sodium chlorite in acidic chloride media

Abstract

Controlling aqueous 5f-element electron transfer chemistry is critical for processing efforts associated with actinide technologies. Often, redox agents are added during actinide processing steps to control actinide redox chemistry and manipulate the actinide oxidation states for the separation. Sodium chlorite, NaClO_2(aq), represents one of these useful redox agents. For example, NaClO_2(aq) finds widespread application in the processing of plutonium and americium. Surprisingly, however, redox reactivity between NaClO_2(aq) and other actinides, like neptunium, has been largely ignored. That knowledge gap is addressed herein. We characterized some redox reactivity between NaClO_2(aq) and Np⁴⁺_(aq) and identified experimental conditions that held neptunium in the +4 oxidation state or converted Np⁴⁺_(aq) to NpO₂²⁺_(aq) or NpO₂¹⁺_(aq). This was achieved by carefully adjusting four variables: ingoing concentrations of (1) Np⁴⁺_(aq), (2) NaClO_2(aq), (3) Cl¹⁻_(aq), and (4) H¹⁺_(aq). We discovered that three neptunium oxidation states (+4, +5, and +6) could be accessed using one ubiquitous redox agent, NaClO_2(aq). These results highlight the diverse electron transfer chemistry available to neptunium in aqueous solutions, provide new insight on how neptunium reacts with NaClO_2(aq), and are discussed within the context of their importance to plutonium and americium processing.